The present invention relates to data storage, and more particularly, to detecting a servo pattern of a magnetic tape using a data channel in a magnetic tape drive.
In a magnetic tape drive, data is typically written and read concurrently with 8, 16, 32, or more data channels. The placement of these data channels is determined by the use of servo patterns that are written onto the tape media by the tape manufacturer. The width of the servo pattern is relatively small compared to the width of the tape and the width of the data tracks, with most of the tape space being allocated to the storage of data in the data tracks.
When the tape starts moving from a stopped position, it is important that the magnetic head (which may include multiple different servo heads or readers, magnetic recording heads or writers, and/or magnetic reading heads or readers) be positioned quickly to a correct location with respect to the servo pattern. In past tape drive designs, the servo system moves the magnetic head around in a systematic manner until the servo heads are positioned over the servo patterns. Once the servo heads are positioned over the servo patterns, the servo system correctly positions the magnetic head on the servo pattern so that data is able to be written and/or read from the data tracks.
One method for using the data channels to help find the servo pattern in a servo acquisition mode has some drawbacks. Firstly, the filter used by the data channel, a 17-tap finite impulse response (FIR) filter, is not sufficient to appropriately filter the servo signals while in the servo pattern detection mode. Since the frequencies involved with the servo pattern are substantially lower than the frequencies of the data channel, the filtering was not adequate to suppress noise when reading servo patterns with the recording heads.
In order to meet noise requirements and design constraints of modern tape drives, the frequency of the sampling clock was changed by re-programming a phase-locked loop (PLL). There is a latency effect when this is performed in that it takes time for the PLL to change to the new frequency.
Secondly, the servo pattern detector only detects servo pattern peaks on one side of the waveform, i.e., either only positive peaks or only negative peaks. However, in actual operation, the servo pattern may be acceptable to detect on one polarity of peaks but not acceptable to detect on the reverse polarity. This results in inconsistent servo pattern detection, latency, and a decrease in performance for the tape drive.